1

Performance Analysis of a Reliable Handoff Procedure Performance Analysis of a Reliable Handoff Procedure Performance Analysis of a Reliable Handoff Procedure Performance Analysis of a Reliable Handoff Procedure forforforfor IEEE IEEE IEEE IEEE 802.16e based networks802.16e based networks802.16e based networks802.16e based networks

V. Rangel1, J. Gómez, E. Cota

Wireless Networks Research Lab National Autonomous University of Mexico

Ed. Valdés Vallejo 3er piso, Telecommunications Department

CU, 04510, México, D.F. E-mail: victor@fi-b.unam.mx

1 The research presented in this paper is founded by the National

Autonomous University of Mexico. Project: UNAM PAPIIT IN110805.

AbstractAbstractAbstractAbstract

IEEE 802.16 is a protocol for fixed broadband wireless access

that is currently trying to add mobility among mobile users in the

standard. However, mobility adds some technical barriers that

should be solved first, this is the case of HO “handoff” (change

of connection between two base stations “BS” by a mobile user).

In this paper, the problem of HO in IEEE 802.16 is approached

trying to maintain the quality of service (QoS) of mobile users. A

mechanism for changing connection during HO is presented. A

simulation model based on OPNET MODELER v11 was

developed to evaluate the performance of the proposed HO

mechanism. Finally, this paper demonstrates that it is possible to

implement a seamless HO mechanism over IEEE 802.16 even for

users with demanding applications such as voice over IP.

IntroductionIntroductionIntroductionIntroduction

A great demand for fast Internet access, voice and video

applications, combined with the global tendency to use wireless

devices, has given sprouting for broadband wireless access

(BWA) networks. Unlike other broadband technologies, such as

xDSL (Digital Subscriber Line), FITL (Fiber In The Loop),

WITL (Wireless In The Loop) among others, BWA networks are

easier to implement and to expand, they require a low initial

investment and a low maintenance cost. In addition, BWA

networks are easy to update and they promise to have a good

future due to the evident growth of the market for broadband

access.

Nevertheless, it was not until last decade that some

international institutions began to make efforts to standardize this

type of technologies. The first attempt of a BWA system was the

Wireless ATM protocol [1], but the lack of industry support led

this system to be an unviable broadband solution for residential

users.

However, a promising solution for broadband wireless access

is with the IEEE 802.16 [2] protocol that was developed at the

beginning of this decade by hundreds of engineers from the

world's leading operators and vendors as well as by many

academic researchers. The first version of this protocol was

standardized on April 2002 and supports data rates up to 134

Mbps in a 28MHz channel, with a 30-mile range. At the

beginning of its development, this protocol was oriented for fixed

wireless users with line of sight (LOS), using the 11-66 GHz

spectrum range. But then on 2004, the aim of this protocol was

changed to support residential access and NLOS. The second

version is called the IEEE 802.16-20004 Standard [3] and

supports two ways of Media Access Control: 1) point to

multipoint (PMP), where traffic only occurs between Base

Station (BS) and Subscriber Stations (SS), and 2) Mesh topology

where traffic can be routed through other SSs and can occur

directly between SSs. In addition, the second version also

includes OFDM modulation and supports 256 carries, which

reduces considerably multipath fading effects.

Recently, the IEEE 802.16 Task Force, released a new version

that will provide mobility to SSs, this is the IEEE 802-16e [4]

standard. Although this standard provides a HO mechanism, it

have not been tested for the support of timing critical interactive

applications, such as Voice over IP. Thus, in this paper we

present a HO mechanism that guarantees QoS for realtime

applications.

The rest of the paper is structured as follows. In section 2,

we present an overview of the IEEE 802.16 MAC protocol and

provide detailed information of the Initialization and Registration

processes that each subscriber station performs with the BS after

power-up. In Section 3 and 4 we describe the proposed HO

mechanism and the simulation model, respectively. Then in

Section 4, we present a performance analysis of the proposed HO

mechanism for different traffic scenarios. Finally in Section 5,

we present a conclusion and future work.

IEEE 802.16 Standard OverviewIEEE 802.16 Standard OverviewIEEE 802.16 Standard OverviewIEEE 802.16 Standard Overview

The IEEE 802.16e standard uses the same Medium Access

Control (MAC) protocol defined in IEEE 802.16 [2], with

several different physical layer specifications dependent on the

spectrum of use and the associated regulations. In general, the

MAC protocol defines both frequency division duplex (FDD)

and time division duplex (TDD). Transmissions from a Base

Station (BS) to Subscriber Stations (SSs) are conducted by a

Downlink (DL) channel, with PMP wireless access using a

frequency channel for FDD or a time signaling frame for TDD.

Multiple SSs share one slotted uplink (UL) channel via TDD on

a demand basis for voice, data, and multimedia traffic. Upon

2

receiving the demand for bandwidth, the BS handles bandwidth

allocation by assigning uplink grants based on requests from SSs.

A typical signaling frame for TDD includes a DL subframe and a

UL subframe. The DL subframe includes one downlink-physical-

protocol data unit (DL-PHYPDU), which includes a preamble,

Frame Control Header (FCH), and a number of data bursts. A

UL-PHY-PDU includes preamble, multiple MAC PDUs, and a

PAD. A MAC PDU includes MAC header, payload, and CRC.

Upon entering the BWA network, each Subscriber Station

(SS) has to go through the Initialization and Registration setup

illustrated in Figure 1.

On power-up, subscriber stations need to synchronize with a

downlink channel (DL-Ch) and an uplink channel (UL-ch).

When a SS has tuned to a DL-ch, it gets the frame structure of

the UL-ch, called a UL-MAP frame as illustrated in Figure 2.

Then the ranging procedure is performed, where the round-trip

delay and power calibration are determined for each SS, so that

SS transmissions are aligned to the correct mini-slot boundary.

The next step is to establish IP connectivity, the Base Station

(BS) uses the DHCP mechanisms in order to obtain an IP address

for the SS and any other parameters needed to establish IP

connectivity. Then, the SS establishes the time of the day, which

is required for time-stamping logged events and key

management. In the next step, the SS establishes a security

association and transfers control parameters via TFTP, these

parameters determine the BS and SS capabilities, such as QoS

parameters, fragmentation, packing, among others. Finally, the

registration process is performed; the SS must be authorized to

forward traffic into the network once it is initialized,

authenticated and configured.

The ranging and automatic adjustment procedure (points A

and B in Figure 1) are considered as the most time consuming

stages, since the first attempt to synchronize with the BS (in a

UL channel) must be done using the initial maintenance region.

This region normally describes two or four contentions

opportunities, every 500 or 1000 ms, (depending on the network

configuration), as show in Figure 2.

Thus the risk of collision is very high when there are more

than two SSs trying to synchronize with the BS at the same time.

In order to guarantee low delays for applications demanding QoS

(e.g.: VoIP), we need to make sure this delay is under 50 ms for

VoIP-quality calls, as described in [5].

Figure 1. Figure 1. Figure 1. Figure 1. Initialization and Registration Procedure.Initialization and Registration Procedure.Initialization and Registration Procedure.Initialization and Registration Procedure.

DL-Frame

Maintenance

Initial Station

Contention Access Reservation Access

UGS rtPS nrtPS BE

Downlink

Channel

(TDM)

Uplink

Channel

(TDMA+

random

acess)

UL-Frame (0.5, 1 or 2 ms Frame)

DL-MAP UL-MAP DL-Data

Request Tx.

Fragmentation

DL-Data DL-Data DL-Data

QPSK 16-QAM 16-QAM 64-QAM

Fragmentation

+Piggyback

Figure Figure Figure Figure 2222. UL. UL. UL. UL----MAPMAPMAPMAP and DLand DLand DLand DL----MAPMAPMAPMAP channel strchannel strchannel strchannel structure.ucture.ucture.ucture.

3

Figure 3 presents the access delay when a SS carries up the

Initialization and Registration process with the BS, for different

UL-MAP structures, represented by the variable “Tmap” in

Figure 3, (e.g. 0.5, 1.0, 1.5, 2.0, and 2.5 ms). In this figure we

appreciate that access delays depend on the total number of

collisions registered in the initial maintenance region. This delay

could be up to 9 seconds, due to the exponential backoff

algorithm and continuous collisions. Thus, in order to support

QoS for mobile users, it is necessary to incorporate a Hand Off

mechanism that could reduce significantly the delay for the

Initialization and Registration process. In other words, mobile

users do not need to go throughout the complete Initialization

process when they want to change the BS, because the serving

BS contains most of the information that the target BS needs to

accept the incoming mobile user. Thus, in this paper we present a

HO mechanist that guarantees very low delay for the support of

realtime applications, multimedia services and high speed

Internet access.

Once the Initialization and Registration setup is complete, a

SS can create one or more connections over which their data are

transmitted to and from the BS. Subscriber stations request for

transmission opportunities on the UL channel. The BS gathers

these requests and determines the number of time slots (grant

size) that each SS will be allowed to transmit in the UL-Frame.

This information is broadcasted in the DL channel by the BS

using the UL-MAP message at the beginning of each DL-Frame,

(see Figure 2). The UL-MAP frame contains Information

Elements (IE) which describe the transmission opportunities in

the UL channel, such as initial maintenance, station maintenance,

contention and reservation access. After receiving a UL-MAP, a

SS will transmit data in the predefined transmission indicated in

the IE, these transmission opportunities are assigned by the BS

using the following QoS service agreements described in [2] and

[6]. These classes of services are described below.

a) Unsolicited Grant Service (UGS): This service is oriented for

the support of real-time service flows that generate fixed- size

data packets on a periodic basis (CBR-like services), such as

T1/E1, VoIP or videoconference.

b) Real-Time Polling Service (rtPS): This service is oriented for

the support of real-time service flows that generate variable

size data packets on a periodic basis (VBR-like services), such

as MPEG video streams. The rtPS service offers periodic

unicast transmission opportunities (utxop), which meet the

flow’s real-time needs and allow the SS to specify the size of

the desired channel reservation.

c) �on Real-Time Polling Service (nrtPS): This type of service

is like the rtPS, however polling will typically occur at a much

lower rate and may not necessarily be periodic. This applies to

applications that have no requirement for a real time service

but may need an assured high level of bandwidth. An example

of this may be a bulk data transfer (via FTP) or an Internet

gaming application.

d) Best Effort (BE): This kind of service is for standard Internet

traffic, where no throughput or delay guarantees are provided.

Enhanced HO Mechanism for IEEE 802.16eEnhanced HO Mechanism for IEEE 802.16eEnhanced HO Mechanism for IEEE 802.16eEnhanced HO Mechanism for IEEE 802.16e

Currently, there are various handoff schemes that has been

proposed for IEEE 802.16e[4]. In this section we present an

enhanced HO mechanism that will enable a Mobile Subscriber

Station (MSS) to change BS without interrupting or degrading its

QoS.

In order to provide a rapid (or transparent) handoff process, it

is necessarily that MSS demanding a real-time service (such as:

UGS for voice o rtPS for video) makes an association with

neighboring BS before a handoff decision is taken. For other

QoS levels, such as Best Effort (for Internet traffic) or non real-

time services (such as FTP traffic) this association could be

optional. In general, the association process can be performed

once the MSS is in normal operation with the serving BS, (this is

after the registration and IP connectivity process). This

association will enable the MSS to speed up the Initialization and

Registration process described in Figure 1.

The association process is carried out as follows:

1) All BS must broadcast “beacons” to inform MSS about their

presence and network topology using a Neighbor Advertisement

Message (NBR-ADV), as show in Figure 4. These beacons help

the MSS to identify neighboring BS and to obtain the DL-MAP

and UL_MAP which include the DCD and UCD settings,

respectively that the MSS needs for a rapid synchronization and

initial ranging.

2) After receiving a NBR-ADV message, a MSS may issue a

Scan Request message (SCN-REQ) to the serving BS in order to

receive a scanning interval for the purpose of seeking and

monitoring neighboring BS (these are the target BS).

3) Upon reception of SCN-REQ, the serving BS should send a

Scan Response message (SCN-RSP) to the MSS, indicating the

start of the scanning interval and its duration.

4) The MSS should use the allocated scanning interval to

synchronize with neighboring BS and compute the quality of the

PHY connection (S/N+I). This interval can be used for

association as well with the neighboring BS, which comprises of

two stages: Association-initial ranging and Association pre-

Figure 3Figure 3Figure 3Figure 3. . . . Initialization and Registration Delays.Initialization and Registration Delays.Initialization and Registration Delays.Initialization and Registration Delays.

FigureFigureFigureFigure 4. Association process.4. Association process.4. Association process.4. Association process.

4

registration

5) Association-initial ranging: This stage is performed by

transmitting a ranging request message (RNG-REQ) with the

following parameters: Basic CID, DL Ch. ID, MSS MAC

Address, MSS Association Ch. ID and the Serving BS ID. This

association will allow the neighboring BS to identify the MSS

and its Serving BS when the HO process is initiated. This

request may also be used to adjust transmission parameters, such

as power, frequency and timing offset, as suggested by the

neighboring BS. Thus, multiple RNG-REQ/RSP may be

exchanged using the same scanning interval.

6) The neighboring BS, upon receiving a RNG-REQ with

association, answers with a ranging respond message (RNG-

RSP) indicating the type of service level that it could support for

this MSS.

7) Association-pre-registration: After receiving a RNG-RSP

which includes some service level, the MSS may pre-register

with the neighboring BS to negotiate MSS capabilities. This state

is called the association pre-registration and some of the

parameters exchanged are: new CID, service flows, backoff

windows for ranging (HO_ranging_start HO_ranging_end),

service level prediction, modulation types (QPSK, QAM), FEC

Types, MAC CRC support, etc.

Once association is performed, the HO process is carried out

as follows (see Figure 5):

The messages between BS must be transmitted using a

dedicated line with high transmission priority, thus reducing at

minimum transmission delay between BS.

1) We assume the worst case, when a user is active and has a

traffic session with its serving BS. Whenever the MSS o BS

detect on the PHY channel a signal with a low quality (e.g S/N+I

< 11.2 dB with QPSK), the HO process is initialized by sending

a HO request message. In Figure 5 (point 1) we show the case

when the MSS initializes the HO process by issuing a HO

request message (MSSHO-REQ) to the serving BS. This

message contains IDs of the neighboring BS, with its respective

S/N+I and service level prediction acquired from the scanning

interval.

In case the serving BS detects a low S/N+I from a registered

MSS, it should send a handoff-BS request notification to the

MSS indicating the recommended neighboring BS and service

level prediction that they could provide to the MSS.

2) Upon receiving a MSSHO-REQ, the serving BS sends a

HO-notification message to each neighboring BS added in the

MSSHO-REQ. This notification includes the MSS ID,

connection parameters, capabilities, required QoS and

bandwidth that a MSS needs to continue normal operation.

3) Then, if the neighboring Base Stations (receiving a HO-

notification) can accept the MSS with its minimum

requirements, they should respond to the serving BS with the

message HO-notification response (ACK) indicating the

estimated QoS and BW that could provide to the MSS.

4) Upon receiving the HO-notification response from the

neighboring BS, the serving BS creates two messages: the first

message is a handoff response to the MSS, (BSHO-response)

that includes the objective BS. This is the best neighboring BS

that could accept the MSS, based on the service level

prediction and S/N+I level (e.g. BS3). The second message is a

confirmation (HO-confirm) to the neighboring BS that has

been selected as the objective BS.

Figure 5. Enhanced HO Process.Figure 5. Enhanced HO Process.Figure 5. Enhanced HO Process.Figure 5. Enhanced HO Process.

5

5) Once the BSHO-response is received, the MSS sends back

to the serving BS a HO indication to close all connections. Then

it should wait for a fast-ranging transmission opportunity from

BS3 in order to re-adjust transmission parameters, such as power,

frequency and timing offset.

6) The objective BS (Target BS3), upon receiving the HO-

confirm message, should provide to the MSS with a fast ranging

transmission opportunity in the following MAPs. The Ranging

IE opportunity is set using the MAC address of the MSS.

7) Since a MSS has already associated with BS3, the MSS will

send again RNG-REQ to BS3 (to re-adjust transmission

parameters in case it is necessary), using the Ranging IE

opportunity provided by BS3, (the MSS uses its MAC address to

identify the ranging opportunity).

8) The MSS should complete initial network entry with target

BS3. For this it is necessary to perform re-authorization, re-

register, re-establish connections, and re-establish IP

connectivity. In the re-authorization state, since the security

information in not expected to change, this information is

transferred from the old BS1 to the target BS3 via backbone. The

re-register process is skipped if all MSS capabilities and

management parameters were exchanged in the pre-registration

process and a service level prediction was provided to the MSS.

In the re-establish connection (service flows) process, the MSS

needs to send a dynamic service addition request (DSA-REQ) to

target BS3, in order to active the service flow. As a response,

target BS3 activate the connection, and maps this service flow

with the MSS CID (provided in the pre-registration process) and

sends back to the MSS a DSA-RSP message, indicating the

service flow activated.

Other stages, such as transfer-operational-parameters and

time-of-day establishment (carried out on MSS power on) are

skipped since none of the operational parameters nor time-of-day

is expected to change.

The last process is to re-establish IP connectivity. This process

should be carried out using Mobile IP as suggested in [7]. This is

because when an MSS which has obtained an IP address by

DHCP performs the inter-BS handoff, an ongoing session is

disconnected due to the inherent lack of mobility of DHCP

address allocation. The time taken to obtain the IP address is out

of the scope in this paper. In the following sections we describe

the simulation model and present a performance analysis on the

enhanced HO mechanism, for different traffic scenarios.

OPNET Model DescriptionOPNET Model DescriptionOPNET Model DescriptionOPNET Model Description

This section shows the key functional breakdown of the

OPNET model implementation. The model used for the

simulations consist of three cells, as shown in Figure 6. Each cell

can be partitioned into two major parts: 1) Mobile Subscriber

Stations and 2) a Base Station, as shown in Figure 6.

Mobile Subscriber Station

The Mobile Subscriber Station access node consists of a traffic

generator, a Media Access Control unit and two RF modules for

transmission and reception as pointed out in Figure 7.

The traffic generator module produces three different traffic

types according to the QoS agreed with the BS. These traffic

types are described in the following section.

Figure Figure Figure Figure 6666. Network Model. Network Model. Network Model. Network Model....

Figure Figure Figure Figure 7777. SS access node model. SS access node model. SS access node model. SS access node model....

6

The RF modules (ant_rx, from_link_rx, to_link_tx, and

ant_tx) are responsible for accepting packets from or transmitting

packes to the radio access network according to the propagation

model described in [8].

The MAC module is responsible for processing packets of

higher layers and transmitting packets to the radio access

network according to its QoS level. The MAC module uses a

primary Finite State Machine (FSM) and a secondary FSM. The

primary FSM (Figure 8) is responsible of the Initialization and

Registration procedure as well as the reception and processing of

synchronization packets, UCD, UL-MAP and DL-MAP frames

from the BS. All packets received from higher layers, are

processed at the application traffic process and sent to the

secondary FSM for transmission on the UL channel, as shown in

Figure 9.

Upon receiving a traffic packet from the Primary FSM, the

secondary FSM processes this packet according to its QoS level.

If the packet is for a Best effort service (e.i. Internet packet), the

transmit opportunity state (Tx_Opp_Proc) looks for a contention

opportunity (either in the current or in the following UL-MAP

frame) and transmits a reservation request to the BS. In case this

request results in collision with other contention transmissions,

the collision resolution (Collision_Res) state takes care to resolve

it according to the exponential backoff algorithm. If the packet

demands a UGS or rtPS service (i.e. for voice or video packets,

respectively) the transmit opportunity state sends a Dynamic

Service Addition (DSA) request to the based station, indicating

its type of service needed for this connection. If this request is

accepted, the No_Request_Outstanding process takes care of

receiving the corresponding grants and to indicate to the

Tx_Opp_Proc when to transmit these voice or video packets.

For nrtPS packets (e.g. for FTP traffic), the same procedure is

carried out as in the previous case, with the exception that the

Request_Outstanding state is now responsible for receiving the

grants from the BS. But if no-grants are allocated for this service,

these packet can still be transmitted in the radio access network

using a Best Effort service.

Base Station

The BS is in charge of MSS´s identification and to provide a

QoS level. The BS also serves as the main gate for incoming and

outgoing packets. Figure 10 shows the BS node model, which is

composed mainly of a MAC unit and one transmission and

reception module for each neighboring cell associated.

The MAC module is responsible of providing to MSS´s with

the right QoS level, and guarantees the correct transmission

opportunities. In order to provide these transmission

opportunities, the MAC module uses also two FSMs.

Basically the primary FSM, illustrated in Figure 11, performs

the following three functions: 1) takes care of the Initialization

and Registration procedure, which is done by the ranging,

rng_rcvd and rng_complete states. 2) Based on MSS´s request,

Figure Figure Figure Figure 8888. Primary SS´s. Primary SS´s. Primary SS´s. Primary SS´s FSM.FSM.FSM.FSM.

Figure Figure Figure Figure 9999. Secondary SS´s. Secondary SS´s. Secondary SS´s. Secondary SS´s FSM.FSM.FSM.FSM.

Figure Figure Figure Figure 10101010. . . . BBBBS node modelS node modelS node modelS node model....

7

the Primary FSM creates the signaling MAPs, which describe the

maintenance region (using the Mtn_MAP state), as well as

contention and reservation access (using the MAP-Time state).

3) Provides with synchronizations and UCD information to

MSSs.

All frames produced in the primary FSM, are passed to the

secondary FSM, which takes care of transmitting theses frames

in the correct DL channels as shown in Figure 12.

Simulations Analysis Simulations Analysis Simulations Analysis Simulations Analysis

In all simulations, one UL channel with a capacity of 2.8

Mbps and one DL channel with a capacity of 22.4 Mbps were

used. For the analysis, we have considered the simulation

parameters given in Table I. Two traffic scenarios were

considered for the results. These scenarios are based on the

following traffic sources.

1) Internet Traffic-IP: The Internet traffic distribution utilized

is the one introduced by the IEEE 802.14 working group [9]. The

message size distribution is as follows: 64-byte Pk. 60%, 128-

byte Pk. 6%, 256-byte Pk. 4%, 512-byte Pk. 2%, 1024-byte Pk.

25% and 1518-byte Pk. 3%. The inter-arrival times are set in

such a way that the Internet offered load per active station is 38.4

kbps at the PHY layer.

2) VoIP- G.723-UGS. This traffic type emulates a speech

codec “G.723.1”, which according to the ITU, IETF and the

VoIP Forum is the preferred speech codec for Internet telephony

applications. This codec generates a data rate of 5.3 kbps or 6.3

kbps depending on the mode, where 20-byte data packets are

generated and encoded every 30 ms. By adding all headers as

illustrated in Table II, one obtains a VoIP stream of 38.4 kbps at

the PHY layer.

For the performance analysis of the proposed HO mechanism,

we need to make sure the end-to-end delay during the handoff

procedure does not exceed 50 ms for timing critical interactive

services. Internet traffic or other delay-tolerant applications can

support larger delays (up to 1second), thus in the remaining of

this sections we will prove that our HO mechanism satisfy the

requirements for the support of different QoS agreements.

Figure 13 and 14 show the results when Internet traffic is being

transmitted. We considered a network size of 40 MSS which

produce 70% of the UL channel capacity. In Figure 13, we see

the access delays of a MSS that started the HO procedure (from

BS1 to BS2) at the simulation time of 54.6 seconds. When the

mobile station switched to BS2, the time consumed during the

HO procedure was 12 ms approximately (6 UL- MAPs). This

delay could be larger depending on the DHCP protocol, which

was not included in the simulations. Once the MSS completed

the Initialization and Registration procedures with BS2, the first

packet transmitted in this cell had a total access delay of 15.5 ms,

which is a low delay for Internet traffic. However, larger delays

Figure Figure Figure Figure 11111111. Primary . Primary . Primary . Primary BSBSBSBS´s´s´s´s FSM.FSM.FSM.FSM.

Table I. Table I. Table I. Table I. Simulation ParametersSimulation ParametersSimulation ParametersSimulation Parameters

Parameter Value

UL data rate (QPSK.) 2.816 Mbps

DL data rate (16-QAM) 22.4 Mbps

Minimum contention slots per UL-Frame 8 slots UL minislot size 16 bytes

UL-Frame Duration 2 ms =44minislots

Simulation time for each run 60s Distance from nearest/farthest SS to the BS 0.1 – 2.35 km

Reed Solomon (short grants/long grants) 6 bytes/ 10 bytes

Limit between short and long grants 245 bytes Maximum number of users in the network 200

Table Table Table Table II. II. II. II. VoIP Codec (G.723.1.VoIP Codec (G.723.1.VoIP Codec (G.723.1.VoIP Codec (G.723.1.

Frame/layer G.723.- 5.3 kbps

Frame size [ms] 30

Voice frame [bytes] 20

RTP [bytes] 12

UDP [bytes] 8

IP [bytes] 20

LLC [bytes] 3

SNAP [Bytes] 5

Ethernet MAC [bytes] 18

IEEE 802.16 MAC 6

PHY: (Prea+GB+FEC) 10+FEC

Total PacketSize 86 bytes or G=9 slots

Net rate at MAC / PHY 22.9 / 38.4 kbps

Prea = Preable, GB = Guarband, and FEC = 6* No_CodeWords

Figure Figure Figure Figure 12121212. . . . SecondarySecondarySecondarySecondary BSBSBSBS´s´s´s´s FSM.FSM.FSM.FSM.

8

(up to 100 ms) were obtained during the simulation, but these

delays are due to collisions in the contention region and the

exponential backoff algorithm which reduces the contention

transmissions opportunities for backlogged MSS.

In terms of throughput, Figure 14 shows that the effect of

changing to another BS is transparent to the MSS, since neither

throughput nor access delays suffered degradation. The average

throughput reported by this simulation was of 32 kbps, which

indicates that all packets were transmitted on time.

Results for VoIP traffic are presented in figures 15 and 16.

In Figure 15 we appreciate a constant access delay of

approximately 34 ms when the MSS moves from BS1 to BS2.

This is because the MSS can not transmit the current VoIP frame

during HO, and this voice frame is kept in the MSS buffer and

transmitted when BS2 allocates a new periodic reservation

opportunity. Thus, at the time of transmitting this voice frame,

the MSS has already received another VoIP frame, which will be

scheduled after its periodic grant interval agreed with BS2

(configured at 30 ms).

However in Figure 16, we see a small reduction in throughput

during the HO procedure due to the buffered packet. But this

reduction gradually disappears as the MSS transmits more voice

frames in BS2.

In general, although the proposed HO mechanism can support

delays under 50 ms for timing critical applications, these access

delays could be significantly reduced if we consider the

following considerations: 1) delete packets during HO for

realtime applications as long as it does not become > 3%, 2)

schedule more reservations opportunities after the HO procedure,

and 3) use the periods of silence for the transmission of buffered

packet. We are currently studying these functionalities and

results will we presented in future publications.

In addition, we did not include a discussion about the impact

of channel errors in the previous analysis. Channel errors can

degrade the QoS observed by MSS in various ways depending of

the particular service class being considered. We are also

investigating ways to overcome this problem within the proposed

HO mechanism.

CCCConclusiononclusiononclusiononclusion

In this paper we have presented the performance analysis of a

HO mechanism for IEEE 802.16e based networks. The proposed

algorithm is practical, compatible with IEEE QoS requirements,

and easy to implement. This mechanism provides a fast

Initialization and Registration procedure that guaranteed low

delays for timing critical applications, such as Voice over IP,

video and data. The performance of this HO mechanism with

mixed traffic sources, and different QoS requirement (such as

UGS, rtPS, nrtPS and BE) will be further investigated through

simulations and theoretical analysis. The results of such

performance will be provided in future publications.

ReferencesReferencesReferencesReferences

[1] B. Bing, “High-Speed Wireless Atm and Lans”, Artech

House Mobile Communications Library, Norwood, 2000.

Figure 1Figure 1Figure 1Figure 13333 Access Delay for BE trafficAccess Delay for BE trafficAccess Delay for BE trafficAccess Delay for BE traffic....

Figure Figure Figure Figure 14.14.14.14. Throughput Throughput Throughput Throughput for for for for InternetInternetInternetInternet traffictraffictraffictraffic....

Figure Figure Figure Figure 15151515 Access Delay for Access Delay for Access Delay for Access Delay for VoIPVoIPVoIPVoIP traffictraffictraffictraffic....

Figure Figure Figure Figure 16.16.16.16. Throughput Throughput Throughput Throughput for for for for VoIPVoIPVoIPVoIP traffictraffictraffictraffic....

9

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